UNIX Tutorial Five

5.1 File system security (access rights)

You will see that you now get lots of details about the contents of your directory,
similar to the example below.

Each file (and directory) has associated access rights, which may be found
by typing ls -l. Also, ls -lg gives additional information
as to which group owns the file (beng95 in the following example):

-rwxrw-r-- 1 ee51ab beng95
2450 Sept29 11:52 file1

In the left-hand column is a 10 symbol string consisting of the symbols d,
r, w, x, -, and, occasionally, s or S. If d is present, it will be at the left
hand end of the string, and indicates a directory: otherwise - will be the starting
symbol of the string.

The 9 remaining symbols indicate the permissions, or access rights, and are
taken as three groups of 3.

The left group of 3 gives the file permissions for the user that owns the
file (or directory) (ee51ab in the above example);

the middle group gives the permissions for the group of people to whom
the file (or directory) belongs (eebeng95 in the above example);

the rightmost group gives the permissions for all others.

The symbols r, w, etc., have slightly different meanings depending on whether
they refer to a simple file or to a directory.

Access rights on files.

r (or -), indicates read permission (or otherwise), that is, the presence
or absence of permission to read and copy the file

Access rights on directories.

w means that users may delete files from the directory or move files into
it;

x means the right to access files in the directory. This implies that you
may read files in the directory provided you have read permission on the individual
files.

So, in order to read a file, you must have execute permission on the directory
containing that file, and hence on any directory containing that directory as
a subdirectory, and so on, up the tree.

Some examples

-rwxrwxrwx

a file that everyone can read, write and execute (and delete).

-rw-------

a file that only the owner can read and write - no-one else
can read or write and no-one has execution rights (e.g. your
mailbox file).

5.2 Changing access rights

chmod (changing a file mode)

Only the owner of a file can use chmod to change the permissions
of a file. The options of chmod are as follows

Symbol

Meaning

u

user

g

group

o

other

a

all

r

read

w

write (and delete)

x

execute (and access directory)

+

add permission

-

take away permission

For example, to remove read write and execute permissions on the file biglist
for the group and others, type

% chmod go-rwx biglist

This will leave the other permissions unaffected.

To give read and write permissions on the file biglist to
all,

% chmod a+rw biglist

Exercise 5a

Try changing access permissions on the file science.txt and
on the directory backups

Use ls -l to check that the permissions have changed.

5.3 Processes and Jobs

A process is an executing program identified by a unique PID (process identifier).
To see information about your processes, with their associated PID and status,
type

% ps

A process may be in the foreground, in the background, or be suspended. In
general the shell does not return the UNIX prompt until the current process
has finished executing.

Some processes take a long time to run and hold up the terminal. Backgrounding
a long process has the effect that the UNIX prompt is returned immediately,
and other tasks can be carried out while the original process continues executing.

Running background processes

To background a process, type an & at the end of the command
line. For example, the command sleep waits a given number of seconds
before continuing. Type

% sleep 10

This will wait 10 seconds before returning the command prompt %. Until the
command prompt is returned, you can do nothing except wait.

To run sleep in the background, type

% sleep 10 &

[1] 6259

The & runs the job in the background and returns the prompt
straight away, allowing you do run other programs while waiting for that one
to finish.

The first line in the above example is typed in by the user; the next line,
indicating job number and PID, is returned by the machine. The user is be notified
of a job number (numbered from 1) enclosed in square brackets, together with
a PID and is notified when a background process is finished. Backgrounding is
useful for jobs which will take a long time to complete.

Backgrounding a current foreground process

At the prompt, type

% sleep 100

You can suspend the process running in the foreground by holding down the [control]
key and typing [z] (written as ^Z) Then to put
it in the background, type